Carrier repeater system



March 24, 1936. H. s. BLACK CARRIER REPRATRR SYSTEM Filed April e, 1934l ATTORNEY Patented Mar. 24, 1936 UNITED STATES PATENT OFFIEE CARRIERREPEATER SYSTEM Application April 6, 1934, Serial No. 719,257

1 Claim.

The present invention relates to the two-way transmission of signalsover separate transmission lines and more particularly to theapplication of repeaters to this type of transmission.

An object of the invention is a simplification and improvement intwo-way transmission of signals over separate lines with amplifiers.

The so called four-wire transmission known in the prior art involvestransmission in the eastward direction over one line and transmission inthe westward direction over a separate line. Each of these lines maycontain one-way repeaters spaced at appropriate intervals. Since theserepeaters only amplify in one direction, no separation between frequencybands, such as bands used in opposite directions, need be made at therepeater points as would be the case in a two-way, two-wire repeater,for example, using carrier waves in different frequency ranges foropposite directions of transmission over the same line. Except forcross-talk difficulties, the same frequency range can be used on bothlines of a fo-urwire system.

In applying multiplex carrier transmission to cable circuits, cross-talkor other difculties may, in some cases, make it necessary or desirableto transmit different bands of frequencies on lines that aretransmitting in opposite directions. The use of separate repeaters inthe individual lines to amplify the different frequency ranges makes therepeater cost run high particularly as such auxiliary apparatus asattenuation equalizers and gain control equipment must be duplicated atsome or all of the repeater points on both lines.

The present invention secures the advantages of four-wire transmissionwith a saving of about half the number of repeaters that would berequired if separate repeaters were used in the individual lines. Whilethis saving is offset to some extent by the necessity of using filtersin connection with the repeaters for separating the two directionalgroups, these filters can be of relatively simple construction andfairly inexpensive since the requirements placed upon them by thecircuit arrangement used in accordance with the invention are notparticularly severe.

A better and more complete understanding will be had of the inventionfrom the following detailed specification taken in connection with theaccompanying drawing.

Fig. 1 is a simplified diagrammatic showing of a two-way carriertransmission system in accordance with the invention;

Fig. 2 shows a modification in the repeater and filter arrangement ofFig. 1; and

Fig. 3 shows a further modification.

Referring to Fig. 1 two telephone lines I and 2 representative of alarger group of lines, are shown connected for two-way repeating withthe lines 3 and 4 through the medium oi' terminal carrier apparatus. Themultiplex carrier wave transmission line 3 terminates in a transmittingbranch 5 leading to individual transmitting channel filters of which twoare shown at 8 and 9. There will be as many of these channel filters asthere are low frequency lines I, 2 to be connected with the multiplextransmission system.

The multiplex line 4 terminates in a receiving branch 6 which leads toindividual l receiving channels of which two, at I Il and I I, areshown.

Telephone line I is provided with the usual hybrid coil I2 and balancingnetwork, the transmitting branch of which leads to modulator I3 which isassumed to contain a source of carrier frequency waves to be modulatedby the speech waves on the line I. The other branch o-f the hybrid coilleads to detector or demodulator I4. Line 2 is similarly connectedthrough carrier transmitting apparatus including modulator I5, andcarrier receiving apparatus including demodulator I6.

The terminal carrier circuit, as disclosed, operates to modulate thecarrier wave sources associated with modulators I3 and I5 With speechwaves received over the individual lines I and 2. A side-band resultingfrom the operation of modulator I3 is selectively transmitted throughthe filter 8 to the transmitting branch 5 and a sideband resulting fromthe operation of modulator I5 is selectively transmitted through thefilter 9 to the same transmitting branch 5. Other lines, not shown,would operate similarly to impress on the transmitting branch 5 asuccession of sidebands spaced in frequency, all of these side-bandsoccupying a relatively high frequency range within the transmission bandlimits of the high frequency filter included in the first repeatersection of the transmission line 3.

The receiving branch 6 receives from the line 4 a similar succession ofside-bands spaced in frequency, all of these side-bands occupying alower range of frequencies within the transmission band limits of thelow frequency filter 2| shown located in the first repeater section ofthe line 4. These various side-bands as received from the branch 6 areseparated by the individual line filters Ill, I I, etc. and impressed onthe individual demodulators I4, I6, etc. by which they are demodulatedto produce voice currents that are then impressed on the` individuallines I, 2, etc.

Associated with the lines 3 and 4 at intervals along the line is asuccession of repeaters 30, 40, etc. arranged to amplify the waves onone line and impress them on the opposite line. The successive repeatersare reversed in direction as shown. These repeaters may be thought of asdividing the lines into repeater sections. Considering the line 3 eachrepeater section contains a filter preferably located near the center ofthe respective section and these filters alternate between highfrequency filters, similar to 20, and low frequency filters similar to 2I. The line 3, for example, is shown as including in successive repeatersections, high frequency filter 20, low frequency filter I2I and highfrequency filter 220. Similarly, the line 4 is shown as including insuccessive repeater sections, low frequency filter 2|, high frequencyfilter |20 and low frequency filter 22|.

In operation, the high frequency groups of waves impressed on the line 3from the transmitting branch 5 are selectively transmitted through thehigh frequency filter 20 and then along the line 3. These waves can notpass through the low frequency filter I2 I, but they are permitted topass into the input of the amplifier 30 where they are amplified andimpressed on the line 4. These waves are prevented from passing backinto the receiving terminal circuit 6 since they will not pass the lowfrequency filter 2|. They do, however, traverse the line 4 toward theright and are allowed to pass through the high frequency filter |20.After traversing this repeater section they pass into the input ofrepeater 40 and are amplified by this repeater and impressed on the line3. Progress is then to the right through the high frequency filter 220to the next repeater station or to the terminal station (not shown) atthe right cooperating with the terminal station shown at the left. l

Waves traversing the lower line 4 from the terminal station (not shown)assumed to be at the right of the figure, are waves comprised in a lowerfrequency group. These waves are selectively transmitted through the lowfilter 22| to the input of amplifier 40 where they are amplifiedandimpressed on line 3. They traverse the next section of line 3 includinglow frequency filter I2| and after arriving at the next repeater stationare impressed on the input of repeater 30 where they are amplified andimpressed on line 4. Traversing the last repeater section of this linetoward the left they are selectively transmitted through the low filter2| and eventually to the receiving terminal circuit 6.

It is thus seen that each repeater 30, 40, etc. is utilized over theentire frequency range transmitted over both lines 3 and 4. Theserepeaters are preferably, though not necessarily, of the type disclosedand claimed in an application by H. S. Black, Serial No. 606,871, filedApril 22, 1932. They are preferably of the stabilized feed-back typedisclosed and claimed in that application by which is meant that theyhave a feed-back path, usually Ygain-reducing, from the output to theinput, resulting, in accordance with the disclosure of that application,in greater stability of operation and in markedly less distortion. Asfurther disclosed in said application, these amplifiers preferablyinclude in the feed-back circuit an attenuation equalizer 3|, 4 I, etc.which unequally attenuates components of different frequencies in thefeed-back circuit in a manner similar to the unequal attenuation of thetransmission lines 3 and 4 over the frequency range.

Fig. 2 differs mainly from Fig. 1 in the provision of four filters perrepeater. One pair of filters, high filter 22 and low filter |24 areassociated with the input of repeater 30, whereas low filter 24 and highfilter |22 are associated with its output. Similarly, in connection withrepeater 40, high filter |23 and low filter 224 are associated with itsinput while low filter |25 and high filter 222 are associated with itsoutput. The waves coming from the left in Fig. 2 on the line 3 passthrough the gain control network 35, high pass filter 22 and areprevented by low filter |24 from` going toward the right in the line 3.These waves are amplified at 30 and are prevented by the low filter V24from going toward the left in the line 4. These waves are selectivelypassed by the high filter |22 to the repeater section on the right. Atthe next repeater station they pass through the gain adjusting network3'I, land high pass filter |23 to the input of amplifier 40, etc.

Similarly, low frequency waves on the line 4 coming in at the right-handside of the figure, pass through gain adjusting network 38, low passfilter 224 to the input side of repeater 40 and thence through therepeater and the low filter |25 to the outgoing line section. At thenext repeater station they pass through the gain adjusting network36,*low pass filter |24 to the input of amplifier 30, etc.

It will be noted that the gain adjusting networks 35 and 37 areindividual to line sections which transmit only waves in a highfrequency group. Similarly, the gain adjusting networks 36 and 38 areindividual to line sections which transmit only the waves in the lowfrequency group. Independent adjustment may be made in each of thesenetworks to suit the particular conditions.

In repeater 30 the attenuation equalizer 3| is shown as in the case ofFig. 1 and in addition there is variable network 32. Similar elements 4Iand 42 are associated with the repeater 40. The variable elements 32 and42 may be adjustable gain control pads in the feed-back circuit of theamplifier. It will be understood that similar variable gain control padsmay be associated with the repeaters in Fig. 1. By the use of theseadjustable networks the gain of the repeater may be varied from time totime either manually or automatically by known methods to take care ofvariations in line attenuation. The individual pads 35, 36, 31 and 38may then be used as supplemental gain adjusting devices of simpleconstruction and with a relatively narrow range of adjustment. In fact,it will probably not be necessary to supply these supplemental networksat every repeater station. Attenuation compensating networks individualto the line sections may also be included similarly to the networks35,36, 31 and 38, or may be incorporated as a part of these networks or asapart of the filters 22, |24, |23, 224, etc. Thus, the attenuationequalizers 3| and 4I may effect the principal attenuation equalizationand a supplemental network located in the line adjacent the input to therepeaters may be used to give the rest of the equalization that isrequired.

It was noted above that the circuit of Fig. 1 by virtue of its simplearrangement represents an economy of apparatus. With the filters in themiddle of the repeater sections the level diiferin lter construction.Also each illter is terminated by reasonable impedances, which alsomakes for low cost filter construction, The repeaters have no rigidimpedance requirements to meet, the main requirement being suicientlyhigh input and output impedance, which can be readily obtained byrepeater designs of types disclosed in applicants copending applicationabove referred to.

A limitation to the circuit of Fig. 1 is the crosstalk which may arisefrom the fact that the high level output currents are allowed totraverse half of a repeater section in a direction opposite to theiruseful direction of transmission and because of this they may cross-talkinto the adjacent oppositely-directed line. For example, 10W frequencycurrents amplified at 3U in Fig. 1 pass to the right on line 4 untilthey reach lter |20 and in so doing they may cross-talk into the sectionof line 3 immediately above in the figure.

A circuit which avoids this cross-talk tendency but offers economy ofdesign is the circuit shown in Fig. 3. This is similar in circuitconfiguration to Fig. 2 with the input filters omitted. For example,repeaters 30 and 40 have no lters adjacent their input side, but retainoutput grouping filters as in Fig. 2, these being filters 24 and |22 inthe case of repeater 30 and lters |25 and 222 in the case of repeater40. The individual filters may differ in loss characteristics from thoseof Fig. 2. For example, they may have loss characteristics similar tothe lters of Fig. 1 or such other characteristics as may be required inparticular cases. The near-end cross-talk (e. g. that arising from thehigh frequency currents output from repeater 30 through filter |22crosstalking into the section of line 3 immediately above in the figure)can be kept sufficiently low in any practical case by keeping therepeater gain sufficiently low, which in turn requires a sumcientlyclose repeater spacing. 'Ihe nearend cross-talk is generally so low thata practical margin remains for repeater gain, leaving the requisiterepeater spacing at a practical value,

say 10 to 25 miles in the case of a typical cable installationtransmitting up to the order of 40 kilocycles.

'Ihe showing of the feed-back paths for the repeaters is to be taken asillustrative and not as limiting since any suitable manner ofassociating this path with the repeaters and any suitable path may beused. The copending application referred to above discloses a number ofways of associating feed-back paths with amplifiers, as by connectingthe feed-back paths between the input and output transformers and therespective input and output lines, or ori the amplifier side of suchtransformers, or using a bridge circuit on one or each end of thefeed-back paths, or still other ways. It is contemplated in the practiceof the present invention to resort to any of those or other suitabletypes of circuit.

It will be understood that the invention is not limited to the specificcircuits and apparatus that have been described, but modifications maybe made within the spirit and scope of the appended claim.

What is claimed is:

In a wave transmission system two lines, means to impress waves in a lowrange of frequencies on one line, means to impress waves in a high rangeof frequencies on the other line at the opposite end thereof, said linesbeing divided into sections by repeaters bridged across from one linet-o the other, and lter means directing the waves of each range throughthe first section of its respective line, thence through the respectiverepeater at the end of that section, thence through the next section ofthe opposite line, and v l so on, said repeaters comprising feed-backamplifiers each containing in the feed-back loop an attenuationequalizer which gives the amplifier different gain at differentfrequencies such as to compensate for unequal attenuation on both of thetwo line sections to which the repeater input is connected for therespective frequency ranges utilized on said sections HAROLD S. BLACK.

